Central South Pacific thermocline water circulation from a high-resolution ocean model validated against satellite data: Seasonal variability and El Niño 1997-1998 influence Elodie Martinez, Alexandre Ganachaud, Jérôme Lefèvre, Keitapu Maamaatuaiahutapu To cite this version: Elodie Martinez, Alexandre Ganachaud, Jérôme Lefèvre, Keitapu Maamaatuaiahutapu. Central South Pacific thermocline water circulation from a high-resolution ocean model validated against satellite data: Seasonal variability and El Niño 1997-1998 influence. Journal of Geophysical Research. Oceans, Wiley-Blackwell, 2009, 114, pp.C05012. 10.1029/2008JC004824. hal-00406938 HAL Id: hal-00406938 https://hal.archives-ouvertes.fr/hal-00406938 Submitted on 20 Jun 2014 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, C05012, doi:10.1029/2008JC004824, 2009 Central South Pacific thermocline water circulation from a high-resolution ocean model validated against satellite data: Seasonal variability and El Nin˜o 1997–1998 influence Elodie Martinez,1,2 Alexandre Ganachaud,3,4,5 Jerome Lefevre,3,4 and Keitapu Maamaatuaiahutapu1 Received 21 March 2008; revised 12 January 2009; accepted 28 January 2009; published 13 May 2009. [1] The oceanic circulation in French Polynesia, central South Pacific, is investigated based on a high-resolution model validated with satellite data. We focus on the upper 500 m, where ocean current variability has been barely documented due to a lack of in situ measurements. Seasonal and El Nin˜o–Southern Oscillation 1997–1999 variability of surface and subsurface currents are characterized. In the north, the South Equatorial Current (SEC) follows a seasonal cycle with winter enhancement corresponding to trade wind strengthening. During summer, the SEC slows and the eastward South Equatorial Countercurrent (SECC) appears across the domain north of an upward Ekman pumping region related to the South Pacific Convergence Zone development in the area. Around the Marquesas Islands, a wind curl dipole is suspected to create westward and eastward (Marquesas Countercurrent, MCC) jets. The eastward Subtropical Countercurrent (STCC) is found south of 20°S. It is disturbed by eddies and broken into westward and eastward branches. During El Nin˜o 1997–1998, due to the reversed winds in the equatorial band, the SEC weakened, while eastward countercurrents appeared. The SECC reinforced and moved northeastward in summer 1998. During La Nin˜a 1998–1999, following the trade wind enhancement, the SEC strengthened. The countercurrents weakened or disappeared and the SECC moved southwestward in summer 1999. During both El Nin˜o and La Nin˜a events, the STCC eddy activity strengthened, while the MCC and associated westward jet were absent. These variations and their dynamical origins are discussed, and a coherent picture of the French Polynesian current system and its variations is proposed. Citation: Martinez, E., A. Ganachaud, J. Lefevre, and K. Maamaatuaiahutapu (2009), Central South Pacific thermocline water circulation from a high-resolution ocean model validated against satellite data: Seasonal variability and El Nin˜o 1997–1998 influence, J. Geophys. Res., 114, C05012, doi:10.1029/2008JC004824. 1. Introduction seasonal and interannual modulations associated with dis- placement of the South Pacific Convergence Zone (SPCZ). [2] Most South Pacific Ocean studies have been focused Numerical experiment by Wang et al. [2000] suggests that on its western or eastern part, leaving the central South French Polynesia is a region where the thermocline depth Pacific barely documented [Kessler and Taft, 1987; Morris variability is mostly seasonal, except along the SPCZ where et al., 1996; Delcroix and Picaut, 1998; Penven et al., 2005; interannual variations become important. Their sensitivity Kessler, 2006; Fiedler and Talley, 2006]. The region of experiments indicated that those seasonal variations were interest here is that of French Polynesia ([5°–30°S] [160°– ° barely influenced by oceanic equatorial dynamics whose 130 W]), including its five archipelagoes (Figure 1). It is main impacts are north of 5°S. Rather, the region is under influenced locally by the easterly trade winds, with some influence of local Ekman pumping and other unidentified 1Laboratoire Terre-Oce´an, Universite´de la Polyne´sie Franc¸aise, Faaa, South Pacific processes. French Polynesia. [3] Neither ocean currents nor ocean temperature vari- 2Now at UMR7093, UMPC, CNRS, Villefranche-sur-Mer, France. ability are well described because of the lack of in situ 3Laboratoire d’Etude en Ge´ophysique et Oce´anographie Spatiales, measurements. Basin-scale studies using expendable bath- Institut de Recherche pour le De´veloppement, Noumea, New Caledonia. 4 ythermographs (XBTs, which provide temperature down to Also at Observatoire Midi-Pyre´nne´es–Physique-Chimie-Automatique, Laboratoire d’Etude en Ge´ophysique et Oce´anographic Spatiales, Uni- 400–800 m along ship routes) and satellite data identified versite´Paul Sabatier, Toulouse, France. two dominant features in the French Polynesian region 5Also at Joint Institute for the Study of the Atmosphere and Ocean, [Lagerloef et al., 1999; Delcroix et al., 2005]. From 0°S Pacific Marine Environmental Laboratory, NOAA, Seattle, Washington, to 20°S the north limb of the South Pacific gyre flows USA. westward as the South Equatorial Current (SEC), with Copyright 2009 by the American Geophysical Union. stronger velocities during winter and some intermittent 0148-0227/09/2008JC004824 countercurrents during summer. South of 20°S the Subtrop- C05012 1of16 C05012 MARTINEZ ET AL.: CENTRAL SOUTH PACIFIC WATER CIRCULATION C05012 Figure 1. French Polynesian bathymetry (in meters) and main archipelagos. Overlain is the regional ocean modeling system (ROMS) mean sea surface height (SSH) (contours) and the ROMS mean geostrophic surface currents (names with the corresponding arrows are indicated), the South Equatorial Current (SEC), the oceanic convergence zone to the south, and the intermittent (dashed arrow) South Equatorial Countercurrent (SECC). Archipelago names are indicated (larger sizes), as are the islands of Tahiti, Rapa, and Moruroa. ical Countercurrent (STCC) [Merle et al., 1969] flows ical eastward drift that continues the southern branch of the eastward (Figure 1), above a deeper westward flow belong- subtropical gyre. This work also described the 1987 El Nin˜o ing to the SEC. This eastward flow arises from the spread- event with a weakening of the SEC and a strengthening of the ing of the thermocline whose deeper isopycnals slope down eastward flows with the SECC and countercurrents in the toward the poles (westward flow), while shallow isopycnals northern region, and the eastward drift in the south. slope up to the surface (surface eastward flow) [Huang and [5] On the basis of geostrophic and Ekman surface Qiu, 1998]. Those currents are altered by the local wind currents estimated from satellite data from 1993 to 2001 field and by their encounter with islands and archipelagoes. (TP/ERS SSHA and ERS-1 and -2 wind data), Martinez et [4] The first substantial work on circulation in French al. [2007] calculated the mean and variability of surface Polynesia was the Hawaii-to-Tahiti Shuttle Experiment currents in French Polynesia, showing important seasonal (hereafter referred as the ‘‘Shuttle’’) [Wyrtki and Kilonsky, and interannual variations. The present study extends their 1984], which provided repeated temperature and salinity description to the subsurface currents and to higher spatial measurements over the water column from April 1979 to resolution. Because of the large spatial and temporal varia- June 1980 and allowed a description of the mean and tions measured in the surface fields, the coarse distribution variability of the zonal geostrophic currents and transports of hydrographic data is insufficient, and a regional ocean across 150°W from Tahiti (17°S) to Hawaii (21°N). The model is used to complement the surface satellite data. authors identified an intense (weak) transport in the SEC Section 2 presents the regional model. It is followed by a north (south) of 8°S. They also identified an intermittent brief description of the mean surface properties (section 3). eastward countercurrent between 7 and 14°S, the South The seasonal variability of both surface and subsurface Equatorial Countercurrent (SECC) that occasionally formed currents is presented in section 4, along with the El Nin˜o several branches [Eldin, 1983]. Below the SEC and near 6– 1997–98 influence (section 5). The results are summarized 7°S, the Southern Subsurface Countercurrent (SSCC) was and discussed in section 6. found with two branches. Similar work was done during the ‘‘Hydropol’’ program, a series of biannual cruises from 2. Numerical Model and Configuration 1986 to 1989, between Tahiti, Rapa, the Marquesas and Moruroa (Figure 1), which enabled identification of several [6] The regional ocean modeling system (ROMS) used important features between 8°–28°Sand140°–145°W